Modern technological progress requires development of optical elements based on new materials with specific, controllable properties. In particular, the necessary optical element in modern visual display systems is an optically anisotropic film that is optimized for the optical characteristics of an individual display module.
Various polymer materials are known in the prior art for use in the production of optically anisotropic films. Films based on these polymers acquire optical anisotropy through uniaxial extension and coloring with organic dyes or iodine. Polyvinyl alcohol is one commonly used polymer in this application. However, the low thermal stability of films based on polyvinyl alcohol limits their applications. Polyvinyl alcohol based films are described in greater detail in Liquid Crystals—Applications and Uses, B. Bahadur, ed., Vol. 1, World Scientific, Singapore, N.Y., July 1990, p. 101.
Organic dichroic dyes are a new class of materials currently gaining prominence in the manufacture of optically anisotropic films with desirable optical and working characteristics. Films based on these materials are formed by coating a liquid crystal (LC) aqueous solution of supramolecules formed by dye molecules on a substrate surface with subsequent water evaporation. The produced films are imbued with anisotropic properties either by preliminary mechanical ordering of the underlying substrate surface as described in U.S. Pat. No. 2,553,961 or by applying external mechanical, electromagnetic, or other orienting forces to the coating on a liquid crystal substrate material as described in U.S. Pat. Nos. 5,739,296 and 6,174,394.
Liquid crystalline behavior of dye solutions is known. However, exploitation these dye based liquid crystals for industrial applications, such as in liquid crystal displays (LCDs) and glazing, has raised great interest.
Supramolecules form a lyotropic liquid crystal (LLC). Substantial molecular ordering or organization of dye molecules in columns allows use of these supramolecular liquid crystal mesophases to create oriented, strongly dichroic films.
Dye molecules that form supramolecular liquid crystal mesophases are unique. These dye molecules contain functional groups located at the periphery and confers water soluble properties to the dye molecules. Organic dye mesophases are characterized by specific structures, phase diagrams, optical properties and solubility properties as described in greater detail in J. Lydon, Chromonics, in Handbook of Liquid Crystals, (Wiley V C H: Weinheim, 1998), V. 2B, p. 981–1007, incorporate herein by reference in its entirety.
Anisotropic films characterized by high optical anisotropy may be formed from LLC systems based on dichroic dyes. Such films exhibit both the properties of E-type polarizers, due to light absorption by supramolecular complexes, and the properties of retarders and compensators as described in related co-pending application entitled “Compensator for Liquid Crystal Display”, U.S. provisional patent application Ser. No. 60/549,792 filed Mar. 2, 2004, the entire disclosure of which is hereby incorporated by reference. Retarders and compensators are films with phase-retarding properties in spectral regions where absorption is lacking. Phase-retarding or compensating properties of the films are determined by their double refraction properties known as birefringence (Δn):Δn=|no−ne|expressed in terms of as the difference in refractive indices between the extraordinary wave (ne) and the ordinary wave (no). Depending on the orientation of the molecules in a medium and the direction of the propagation, ne and no varies. For example, if the direction of propagation coincides with the optical or crystal axis, ordinary polarization is predominantly observed. Whereas, if light propagates orthogonally or at some angle that is not orthogonal to the optical axis, the light emerging from the medium will separate and decompose into its extraordinary and ordinary wave components.
Usefully, in addition to optical properties, if high-strength dyes are used, films characterized by high thermal and photo stability can also be fabricated.
Extensive investigations aimed at developing new methods of fabricating dye-based films through manipulation of deposition conditions have been described in U.S. Pat. Nos. 5,739,296 and 6,174,394 and published patent application EP 961138. Of particular interest is the development of new compositions of lyotropic liquid crystals utilizing modifying, stabilizing, surfactant and/or other additives to known dyes, thereby, improving film characteristics.
The demands for producing anisotropic film with improved selectivity in different wavelength ranges are ever increasing. Films with different absorbance maxima over a wide spectral wavelength ranging from infrared to ultraviolet regions are needed for a variety of technological areas.
Hence, much recent research attention has been directed to the materials used in the manufacturing of isotropic and/or anisotropic double refraction films, polarizers, retarders or compensators, herein collectively known as optical materials or films, for LC displays and telecommunications applications, such as, for example, but not limited to those described by P. Yeh, Optical Waves in Layered Media, New York: John Wiley &Sons, Inc, 1998 and P. Yeh, and C. Gu, Optics of Liquid Crystal Displays, New York, John Wiley &Sons, Inc., 1999, incorporated herein by reference in their entirety. It has been found that ultra-thin double refraction films can be fabricated using known methods and technologies to produce optically anisotropic films composed of organic dye LLC systems. Recent reports on manufacturing of thin crystalline optically anisotropic films based on disulfoacids of the red dye Vat Red 14 has been described by Lazarev, P. and Paukshto, M., “Thin Crystal Film Retarders,” 2000, Proceeding of the 7th International Display Workshops, Materials and Components, Kobe, Japan, November 29–December 1, pp.1159–1160, as cis- and trans-isomeric mixtures of naphthalenetetracarboxylic acid dibenzimidazole:
This technology permits control of the direction of the crystallographic axis of a film during coating and crystallization of the LC molecules on a substrate such as glass plates. The deposited films are uniform in composition and have high molecular and/or crystal ordering with a dichroic ratio, κd, of approximately 28, making them useful as optical materials or films such as, but not limited to, polarizers, retarders, double refraction materials (e.g., birefringent film) or compensators.
Now referring to FIG. 1, molecularly oriented red dye based films typically exhibit high anisotropy characterized by a large difference in the refractive indices, i.e., Δn=no−ne, that varies 0.4 to 0.9 over wavelengths in the range of approximately 500 to −700 nm. However, their application is limited because the films operate in a narrow region of the visible spectrum, namely the green spectral region, where the dye does not show significant absorption.
Thin, double refraction films that are transparent in the visible region have been prepared based on sodium chromoglycate (DSCG):
The anisotropy of the oriented film from DSCG is not very high. The difference in the refractive indices An is in the range of approximately 0.1 to 0.13. However, the thickness of the films based on DSCG can be varied over a wide range, thus allowing preparation of films with a desired phase-retarding effect despite the low anisotropic characteristics of the film. These films are discussed in greater detail in T. Fiske, et al., “Molecular Alignment in Crystal Polarizers and Retarders”, Society for Information Display, Int. Symp. Digest of Technical Papers, Boston, Mass., May 19–24, pp. 566–569, 2002, incorporated herein by reference in entirety. The main disadvantage in many of these films lies in their dynamic instability, which leads to gradual recrystallization of the LC molecules and anisotropy degradation.
Other anisotropic materials have been synthesized based on water-soluble organic dyes utilizing the above-mentioned technology, see, e.g., U.S. Pat. Nos. 5,739,296 and 6,174,394 and European patent EP 0961138. These materials exhibit high absorbance in the visible spectral region, while advantageous for many applications it limits their application for forming transparent double refraction films.
Thus, there is a general need for films that are optically anisotropic and sufficiently transparent in the regions in which they operate. There exist needs for films for optics that are transparent in the visible range. It is therefore desirable to provide improved methods for synthesizing and preparing anisotropic films. It is also desirable to provide films for optics that are resistant to changes in temperature.